The purpose of the paper is to study the evolution mechanism of hydrochemical field and to promote unpredictable benefits to the living standards of local people and to the local economy in the southern plain area of Pengyang County, Ningxia, China. Based on the understanding of the hydrogeological conditions in Pengyang County, the chemical evolution characters of groundwater in the plain area were analyzed. PHREEQC geochemical modeling software was used to perform a hydrochemical modeling for water-rock interaction and to analyze quantitatively the evolution processes and the forming mechanism of the local groundwater. Geochemical modeling results showed that along path ①, Na+ adsorption played the leading role in the precipitation process and its amount was the largest, up to 6.08 mmol/L, cation exchange was obvious on path ①, while on simulated path ② albite took up the largest amount of dissolution, reaching 9.06 mmol/L, the cation exchange was not obvious along path ②. Some conclusions were summarized according to the modeling results that along the groundwater flow path, calcite and dolomite in the whole simulation showed oversaturated status with a precipitation trend, while the fluorite and gypsum throughout the simulated path were not saturated and showed a dissolution trend. Total dissolved solids (TDS) increased and water quality become worse along the flow path. Dissolution reactions of albite, CO2 and halite, exchange adsorption reaction of Na+ as well as precipitation action of sodium montmorillonite and calcite are the primary hydrogeochemical reactions which resulted in changes of hydrochemical ingredients.